Magnetic resonance imaging is a valuable aid in diagnosing and locating diseased tissue. Certain paramagnetic metal ions influence the MRI images in favorable ways; however, the inherent toxicity of metal ions has hampered their use as contrast agents. Therefore, chelating agents are used to complex the metal ions, producing in some, but not all cases, well tolerated metal chelate compounds.
Porphyrins have been used as chelating agents for paramagnetic metal ions to achieve higher contrast in MRI images of neoplastic tissue. However, the metalloporphyrins used are made water soluble by several (usually four) highly acidic or basic groups on the periphery of the porphyrin ligand. These groups carry dissociable counter ions, which when the metalloporphyrins are dissolved, dissociate and greatly increase the osmolality of the resulting solutions. Elevated osmolality causes discomfort and has been linked to adverse toxic drug reactions in humans.
Desired properties for contrast agents used in MRI include but are not limited to high water solubility, low osmolality and high relaxivity. Relaxivity is a measure of the effectiveness of each metal chelate complex in affecting T.sub.1 and T.sub.2 relaxation times. The greater the relaxivity, in general, the more effective is the metal complex as a contrast agent. The results of relaxivity studies on manganese porphyrins are disclosed in Chi-wan Chen et al., FEBS Lett., 168, 70 (1984) and Seymour H. Koening et al., Magn. Reson. Med., 4, 252, (1987); these references indicate that these compounds have anomalously high relaxivities.